Hydraulic fracturing is a well-stimulation technique in which subterranean rock is fractured by a hydraulically pressurized fracturing fluid typically made by combining water or an aqueous liquid, a hydraulic fracturing proppant (conventionally sand or aluminum oxide), and additive chemicals that modify subterranean flow, subterranean interfacial tension, and/or provide other effects. A hydraulic fracture is formed by pumping the fracturing fluid into a wellbore at a rate sufficient to increase pressure at the target depth to exceed that of the fracture gradient (pressure gradient) of the rock. When the hydraulic pressure is removed from the well, the hydraulic fracturing proppants lodge within the cracks to hold the fractures open. Hydrocarbon compounds such as natural gas and petroleum are recovered via the cracks in the hydrocarbon-containing deep-rock formations. Hydraulic fracturing techniques can be used to form a new well and can also be used to extend the life of an existing conventional oil well.
In recent years the hydraulic fracturing industry has turned to recycling the water that flows back from the subterranean formations after release of hydraulic pressure thereto. Such water is referred to as “produced water.” Produced water is often characterized as having high total dissolved solids, such as at least about 1 wt % total dissolved solids and as much as about 35 wt % total dissolved solids, in addition to any residual fracturing fluid chemicals flowing back from the injection thereof. Stated differently, the dissolved solids in produced water are derived principally from the subterranean reservoir itself. In most cases, a substantial portion of the dissolved solids are ionic (one or more salts). Rather than treat the produced water to remove dissolved solids, it is economically more practical to simply use the produced water with no further treatment prior to use as a fracturing liquid.
Chemical additives including surfactants and polymers have been added to fracturing fluids in hydraulic fracturing processes to increase recovery of hydrocarbon compounds from subterranean hydrocarbon-containing formations by controlling interfacial energy of the fluid with the subterranean features such as various rock types, to control friction caused by the fracturing fluid as it flows within the subterranean formation and through narrow tubulars, to control viscosity of the fracturing fluid, or two or more thereof.
In order to carry the proppant particles used to keep the cracks in the subsurface formation open once they are fractured, the fracturing fluid needs to be able to carry these particles all the way down and into these cracks. One way of doing this is to increase the viscosity of the fracturing fluid. Crosslinking provides one means by which the viscosity of fracturing fluids can be increased.
A problem encountered during hydraulic fracturing is the loss of fluid injectivity in areas of relatively low permeability due to preferential flow of the fracturing fluid into higher permeability areas, sometimes known as “channeling”. Oil bearing strata are usually heterogeneous, some parts of them being more permeable than others. As a consequence, channeling can occur so that the driving fluid flows preferentially through permeable zone depleted of oil (so-called “thief zones”) rather than through those parts of the strata that contain sufficient oil to make oil recovery operations profitable. Difficulties in oil recovery due to high permeability of zones may be corrected by injecting an aqueous solution of an organic polymer and a crosslinking agent into certain subterranean formations where the polymer will be crosslinked to produce a gel, thus reducing the permeability of such subterranean formations to driving fluid (gas, water, etc.).
Crosslinked fluids or gels are now being used in wells under a variety of temperature and pH conditions. Polysaccharide or partially hydrolyzed polyacrylamide-based fluids crosslinked with certain aluminum, titanium, zirconium, and boron-based compounds are used in enhanced oil recovery operations. Such fracturing fluids can encounter a variety of conditions of high temperature and pressure in subterranean formations.
A disadvantage with many of the known crosslinkers is that they can cause an immediate and excessive increase in viscosity of the fracturing fluids to which they are added. Excessive viscosity increase before the fracturing fluid has sufficiently penetrated the subterranean formation increases strain on pumping equipment and/or requires greater energy consumption to pump the fracturing fluids into the subterranean formations. Excessive fracturing fluid viscosity can also increase shear in the pumping equipment, causing degradation of components within the fracturing fluid and leading to degradation in fracturing fluid performance.
A further issue encountered is that produced waters can contain dissolved reactive species such as boric acid and/or borate oxyanions, which can function as crosslinkers for polysaccharides and cause premature crosslinking of hydraulic fracturing fluids comprising polysaccharides and produced waters.
It would be advantageous to provide hydraulic fracturing compositions and methods for use in a variety of different subterranean conditions, which would allow for penetration of low-permeability zones in addition to or instead of thief zones by proppant bearing fluid. It would be further advantageous if such fluids could be used at the high temperatures and pressures found in deep subterranean locations.